Generally, beam steering devices are used to change the direction of a propagating energy source, such as a light beam. Multiple beam steering devices may be used in concert to direct multiple light beams as desired, including directing all of the light beams at the same distant point.
Conventional gimbaled mirror arrangements, for example, serve as beam steering devices and use one or more pivoting mirrors to reflect light in a desired direction. Typically, each gimbaled mirror is associated with a single light source. For large beam applications, however, the space required to accommodate a large mirror and gimbal, and the associated weight of the combined large mirror and gimbal, become significant. Given these size and weight restrictions, it is often impractical to use multiple large gimbaled mirror arrangements to direct energy at a single distant point. And, where gimbaled mirrors are used in a conformal arrangement, so that the gimbaled mirror is behind a protective window on the side of a ship, for instance, the size of the window is often limited by the structural necessity of supporting the window. Muntins, or muntin bars, have been used and extend across a window like a grid to support a series of individual panes of glass or optically transmissive material that together make up the window. However, because these muntins could interfere with light transmission, especially at large reflection angles, smaller windows that do not require muntins have been used. Thus, the size of the window has effectively limited the size of the gimbaled mirror used and the field of regard in conformal arrangements.
Refractive beam steering systems, including optical phased arrays and Risley prism modules, have also been developed. Optical phased array beam steering systems use an array of narrow electrodes that are electronically controlled to create a ramp of diffraction gratings that act like birefringent prisms. These arrays are still in the initial stages of development, though, and are characterized by low fill factors, small apertures, high transmission losses, and limited steering angles.
Risley prism modules typically use a pair of rotatable round wedge prisms to redirect a laser beam by refraction. Each wedge prism refracts the laser beam by a certain refraction angle, and the combination of multiple wedge prisms allows for the laser beam exiting the Risley prism module to be refracted at angles (up to the maximum refraction angle) much greater than the refraction angle created by a single wedge prism. The individual wedge prisms of a Risley prism module are controlled and rotated independently, allowing the laser beam to be steered to any position within a solid angle defined by the maximum deflection angle. To facilitate the independent rotation of the wedge prisms, Risley prism modules include the necessary mechanical and electrical components for rotating the wedges. These components are contained within a prism rotating assembly (PRA) that extends radially outward from the periphery of the round wedge prisms. The thickness of the PRA surrounding these Risley prism modules is substantial, so it is impractical to arrange them in an array because the array would be characterized by low fill factors. That is, the area of the array that would be occupied by the PRA surrounding each Risley prism module diminishes the area of the array that actually transmits light. A further drawback of conventional PRAs is that they are not conducive for use in arrays due to their relatively large outer-diameter-to-optical aperture ratio. In other words, the prior art beam steerers with their annular ring containing drive motors and/or feedback devices result in significantly spaced apart assemblies when combined as sub-apertures in an array. The resulting low fill factor increases component size, weight and—for coherently phased arrays—undesirable side lobes.
Also, Risley prism modules only handle beam steering and do not include the optical components necessary to expand or focus a laser beam from a laser beam source. Risley prisms also tend to be axially thick which in some instances may diminish the ability to steer a beam in a desired direction.
Thus, a need exists in the art for a beam steering module capable of expanding and collimating a laser beam, capable of steering a laser beam over a large field of regard, and capable of being arranged in an array characterized by high fill factors. And there is a need to collectively direct light beams emitted from multiple light beam steering elements and improve the fill factor when combining multiple sub-apertures in an array.